Mobile data and voice communication continues to grow. The increasing popularity of data and voice communication requires that communication needs of a large number of users must be met which are all located within a small area, a case referred to as dense crowd scenario in the art. Typical examples include sports arenas, shopping malls or large office buildings.
In order to increase data transmission performance and reliability, the so-called multiple-input and multiple-output (MIMO) technology may be used in a wireless radio telecommunication system for transmitting information between a base station and terminals of users.
MIMO systems may use multiple send and receive antennas for wireless communication at a base station. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding provided in MIMO systems allows a spectral and energy efficiency of the wireless communication to be increased.
In a so-called massive MIMO system, the base station may include a large number of antennas, for example several tens or even in excess of one hundred antennas with associated receiver circuitry. The extra antennas of the massive MIMO base station allow radio energy to be spatially focussed in transmissions as well as a directional sensitive reception which improves spectral efficiency and radiated energy efficiency.
In order to adapt the transmit signal at each individual antenna of the base station in accordance with the currently active receiving terminals, a base station logic needs information about radio channel properties between the terminals and the antennas.
A pilot signalling scheme can be used for this purpose which allows the base station to set configuration antenna parameters for transmitting signals, so as to focus radio energy at terminals or for receiving radio signals. Thus, focus may mean both phase align contributions with different path length and transmit only in directions that will reach the terminal. In a conventional MIMO system, training sequences may be transmitted from all terminals within the cell and possibly also neighbouring cells in a time slot which is dedicated to the respective terminal. The training sequences need to be orthogonal in order for the base station to identify the configuration parameters for the plurality of antennas for each of the one of the terminals in conventional systems. Orthogonality may be achieved by using time division multiple access (TDMA), code division multiple access (CDMA) or frequency division multiple access (FDMA) technologies or a combination thereof.
In case the MIMO system uses time division multiple access (TDMA), each terminal can transmit a pilot signal in an assigned time slot, which can be received by the antennas and analysed by the base station logic. It will be appreciated that time slots are one example of orthogonal channels, with orthogonality being attained in the time domain. In order to not interfere with each other, a certain time period can be assigned in each system frame where each terminal may transmit its pilot signal. The time slots in which terminals may transmit their pilot signals in combination are also referred to as pilot portion of the frame. The remaining time slots of the frame may be used for downlink (DL) and uplink (UL) data transmission, with the downlink and uplink transmissions being performed in a plurality of time slots which may follow the header of the frame. The pilot signals may each include a training sequence with the pilot signal received at the plurality of antennas of the base station being analysed by the base station logic. Information about a radio channel property of the radio channel between the terminal and the plurality of antennas may be obtained as a result of the analysis. The base station may use the results of the analysis to determine configuration parameters for transmitting signals via the antennas to the respective terminals.
In particular, massive MIMO systems (MaMi) may be deployed in buildings such as office buildings, shopping malls, sports arenas or other areas in which a large density of users can occur. In such environments, a large number of terminals may be located in a cell served by the MIMO base station. The time required for the pilot signalling of the terminals in each frame may increase with the number of terminals. For a large number of terminals, the time required for all terminals to transmit their pilot signals may exceed the available pilot signalling time in each frame. While the pilot signalling time, i.e. the number of time slots allocated to the pilot signalling, may be adjusted dynamically, the transmission of payload data would be negatively effected if the pilot signalling time was increased too much. Therefore, the resources for transmitting pilot signals are limited.
The pilot signals are send from the terminals to the MIMO base station, i.e., in the uplink direction. Therefore, uplink and downlink data transmissions are based on the quality of the uplink pilot signal. If there is interference during the pilot signal transmission, both uplink and downlink would be effected. The interference may originate from neighbour cells. Furthermore, for mobility reasons, the validity of the channel as defined by the configuration parameters is limited.
A new pilot signal needs to be transmitted at regular terms, for example at about every millisecond. Therefore, the transmission of pilot signals requires an considerable amount of resources. In order to keep the ratio between payload and pilot signal overhead large, the number of orthogonal pilot channels needs to be kept as small as possible.